After a wellbore has been drilled, the wellbore typically is cased by inserting lengths of steel pipe (“casing sections”) connected end-to-end into the wellbore. Threaded exterior rings called couplings or collars are typically used to connect adjacent ends of the casing sections at casing joints. The result is a “casing string” including casing sections and connecting collars that extends from the surface to a bottom of the wellbore. The casing string is then cemented in place to complete the casing operation.
After a wellbore is cased, the casing is often perforated to provide access to a desired formation, e.g., to enable formation fluids to enter the well bore. Such perforating operations require the ability to position a tool at a particular and known position in the well. One method for determining the position of the perforating tool is to count the number of collars that the tool passes as it is lowered into the wellbore. As the length of each of the steel casing sections of the casing string is known, correctly counting a number of collars or joints traversed by a device as the device is lowered into a well enables an accurate determination of a depth or location of the tool in the well. Such counting can be accomplished with a casing collar locator (“CCL”), an instrument that may be attached to the perforating tool and suspended in the wellbore with a wireline.
A wireline is an armored cable having one or more electrical conductors to facilitate the transfer of power and communications signals between the surface electronics and the downhole tools. Such cables can be tens of thousands of feet long and subject to extraneous electrical noise interference and crosstalk. In certain applications, the detection signals from conventional casing collar locators may not be reliably communicated via the wireline.
While the invention is susceptible to various alternative forms, equivalents, and modifications, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto do not limit the disclosure, but on the contrary, they provide the foundation for alternative forms, equivalents, and modifications falling within the scope of the appended claims.